1. Field of the Invention
The present invention relates generally to nuclear fuel assemblies and, more particularly, is concerned with a device in the form of a sleeve for protecting a lower end portion of a nuclear fuel rod cladding from debris-induced fretting damage.
2. Description of the Prior Art
During manufacture and subsequent installation and repair of components comprising a nuclear reactor coolant circulation system, diligent effort is made to help assure removal of all debris from the reactor vessel and its associated systems which circulate coolant therethrough under various operating conditions. Although elaborate procedures are carried out to help assure debris removal, experience shows that in spite of the safeguards used to effect such removal, some chips and metal particles still remain hidden in the systems. Most of the debris consists of metal turnings which were probably left in the primary system after steam generator repair or replacement.
In particular, fuel assembly damage due to debris trapped at the lowermost grid has been noted in several reactors in recent years. Debris enters through the fuel assembly bottom nozzle flow holes from the coolant flow openings in the lower core support plate when the plant is started up. The debris tends to become lodged in the lowermost support grid of the fuel assembly within the spaces between the "egg-crate" shaped cell walls of the grid and the lower end portions of the fuel rod tubes. The flowing coolant causes the debris to gyrate which tends to cut through the cladding of the fuel rod. Thus, the damage consists of fuel rod cladding perforations caused by fretting of debris in contact with the exterior of the fuel rod.
One basic approach to solving this problem of debris-induced damage to fuel rods has been to devise a debris filter or trap in the region of the bottom nozzle of the fuel assembly. Although certain designs of debris filters do function to prevent a significant percent of the debris from reaching the region of the lowermost grid, some debris still gets through and can cause fretting failures.
Consequently, a need still exists for a fresh approach to avoidance of debris-induced damage of nuclear fuel rods. The new approach must be compatible with the existing structure and operation of the components of the reactor, be effective throughout the operating cycle of the reactor, and at least provide overall benefits which outweigh the costs it adds to the reactor.